This paper is the result of geological survey engineering.

Objective The Qiongzhusi Formation in the southern Sichuan Basin is abundant in shale gas resources. Multiple horizontal wells have achieved high commercial gas flow rates, showing favorable prospects for exploration and development. The formation has become a key target for natural gas exploration and development in the Sichuan Basin. In the Leshan-Jingyan region, the Qiongzhusi Formation is characterized by intensified tectonic deformation in the west and weakened deformation in the east, accompanied by large variations in burial depth and strong reservoir brittleness heterogeneity. These factors result in significant discrepancies in fracturing performance among different horizontal wells. Therefore, it is critical to accurately characterize the rock mechanical properties and brittleness of the formation across micro- to macro-scales.

Methods This study focuses on the shale of the Qiongzhusi Formation in the Jingyan area. Nanoindentation tests and scanning electron microscopy were conducted to quantitatively analyze the influence of mineral grain types on microscopic rock mechanical characteristics. Triaxial compression experiments were performed to investigate the influence of lithology on macroscopic rock mechanical properties. A brittleness evaluation method based on mineral composition and microscopic brittleness characteristics was developed, and the fractal dimension was introduced to assess the complexity of sample fracture patterns, thereby evaluating the applicability of different brittleness evaluation methods in the study area. Logging interpretation methods of rock mechanics and brittleness were established based on lithology to evaluate the distribution characteristics of brittleness in the study area, and their impact on hydraulic fracture networks was analyzed.

Results (1) Quartz exhibits the highest elastic modulus and brittleness index, whereas clay minerals show the lowest. (2) Silty shale has an elastic modulus approximately 2 GPa higher than that of black shale, a Poisson’s ratio lower by 0.02, and a compressive strength 30 MPa greater. (3) Three brittleness indices are most suitable for evaluating brittleness in the study area: BI_1w developed in this study based on mineral composition and microscopic brittleness characteristics, BI₅ calculated from macroscopic rock mechanical parameters, and BI₁₀ calculated based on the energy evolution process.

Conclusions The rock mechanical properties and brittleness of the study area are primarily controlled by burial depth, with localized abrupt changes influenced by lithology. A strong positive correlation exists between the brittleness index and the volume of the hydraulic fracture network, indicating that rock brittleness plays a controlling role in fracturing effectiveness.